1. Field of the Invention
The present invention generally relates to a multi-beam scanning apparatus that simultaneously outputs a plurality of beams, and an image forming apparatus that writes an image with a multi-beam, and more particularly to a technology capable of detecting and correcting a beam pitch on a scan receiving surface in accordance with a detection result.
2. Discussion of the Background
Among various image forming apparatuses using an electro-photographic system, such as a copier, a printer, a facsimile etc., there is an image forming apparatus capable of enabling a multi-beam light source apparatus to simultaneously irradiate and write a plurality of laser beams on a plurality of lines in order to increase a writing speed. For example, Japanese Patent Application Laid Open No. 9-80331 refers to a multi-beam light source apparatus including a device capable of correcting a beam pitch, which is an interval in a sub scanning direction, on a scan receiving surface by displacing either an optical element disposed in plural light paths so as to permit transmission or reflect a beam or a line image imaging element. Further, Japanese Patent Application Laid Open No. 6-110273 refers to an optical scanning apparatus that includes a beam combining device capable of combining a plurality of laser beams output from a plurality of laser light sources, and a device capable of correcting a beam pitch by inclining a transparent parallel plate inserted at least between one of the laser light sources and the beam combing device.
In addition, Japanese Patent Application Laid Open No. 11-23988 refers to a multi-beam light source apparatus integrally installing first and second light source sections respectively including a plurality of semi-conductor laser diodes and collimate lenses, a supporting member that integrally supports these devices, and a beam combining device that approximates and irradiates light beams output from the first and second light source sections in a holder. Since the above-described multi-beam light source apparatus of Japanese Patent Application Laid Open No. 11-23988 integrally includes a plurality of members, if each of the members is made of different material, a thermal expansion amount of material varies and an optical axis of the laser beam is relatively deviated along with a temperature change, thereby an interval (i.e., a beam pitch) between scanning lines on a scan receiving surface of a photo-conductive surface possibly changes. In such a situation, the beam pitch can be corrected when the technologies of Japanese Patent Application Laid Open Nos. 9-80331 and 6-110273 are applied to.
However, when the former device is applied to, there arises a problem that another beam property (e.g. a beam spot radius) possibly deteriorates. In addition, due to a limit of physical arrangement, the latter device can probably not be applied to the above-described multi-beam light source apparatus.
Further, when a method of combining a plurality of laser beams with a beam combining device is utilized, a change in a beam pitch or scanning line interval on a scan receiving surface frequently arises as a problem along a change in environment and time elapsing when compared with a method in which a semi-conductor laser array is utilized as a light source unit.
To resolve such a problem, below described apparatuses have been proposed. Specifically, Japanese Patent Application Laid Open No. 2000-227563 refers to a multi beam optical scanning apparatus capable of combining laser beams irradiated from a plurality of light sources with a beam-combining prism. In particular, the apparatus adjusts a beam spot position on a scan receiving surface by shifting the beam combining prism along a light path and adjusting its inclination on either a main or sub scanning direction cross sectional planes so as to adjust an irradiation direction of the light beam.
Japanese Patent Application Laid Open No. 10-215351 also refers to a light beam scanning apparatus capable of combining laser beams irradiated from a plurality of light sources with a beam combining prism. In particular, the apparatus adjusts a beam spot position on a scan receiving surface by shifting a cylindrical lens, which forms a line image on a reflection surface of a polygon mirror, in a sub scanning direction, and adjusting an irradiation direction of the light beam. Japanese Patent application Laid Open No. 9-189873 also refers to a multi beam scanning apparatus and method capable of combining laser beams irradiated from a plurality of light sources with a half mirror. In particular, the apparatus adjusts a beam spot position on a scan receiving surface by adjusting inclinations of all of a galvanometer mirror provided on a light path and a light source apparatus, and thereby adjusting an irradiation direction of the light beam.
All of the above-described background technologies detect a change in arrangement of beam spots, which change is caused by a temperature change or time elapsing or the like, on the scan receiving surface, and performs feedback adjustment based upon its detection result.
However, it is sometimes difficult for a system that combines light beams irradiated from a plurality of light sources with a beam-combining device to initially adjust (set) beam spots on the scan receiving surface to a prescribed value, when a light source apparatus (or an optical scanning apparatus) is assembled, because of affection of parts processing, assembling, and adjustment errors or the like.
The inventors are unaware of an optical scanning apparatus capable of readily performing such initial and feedback adjustment.